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Designing the persistence layer
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We ll use the simple place bid use case from the CaveatEmptor application to demonstrate our ideas. This use case states that when a user places a bid on an item, CaveatEmptor must perform the following tasks, all in a single request:
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Check that when the user enters the bid, the amount is greater than any other bids for the item (you can t bid lower than someone else!). Check that the auction hasn t yet ended. Create a new bid for the item.
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If either of the first two checks fails, the user should be informed of the reason for the failure; if both checks are successful, the user should be informed that the new bid has been made. These checks are the business rules. We also have a nonfunctional requirement: if a failure occurs while accessing the database, the user should be informed that the system is currently unavailable (this is an infrastructure concern). Let s see how you can implement this functionality, starting with an overly simple approach.
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10.1.1 Implementing a simple persistence layer
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In the Hello World application of chapter 2, the example program contained simple functions for everything related to persistence. This design doesn t scale well, and using it in larger applications would result in a sprawling, disorganized mess of functions for creating, reading, updating, and deleting entities. In this section, we ll suggest a tidier approach, where you split the persistence layer into a number of classes, each responsible for a specific concern. The first thing we want to tackle is finding a way for the application to obtain new ISession instances. For this, you ll write a simple helper (or utility) class to handle configuration and initialization of the ISessionFactory (see chapter 3) and also to provide easy access to new ISessions. The full code for this class is shown in listing 10.1.
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Listing 10.1 A simple NHibernate helper class
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public class NHibernateHelper { public static readonly ISessionFactory SessionFactory; static NHibernateHelper() { try { Configuration cfg = new Configuration(); SessionFactory = cfg.Configure().BuildSessionFactory(); } catch (Exception ex) { Console.Error.WriteLine(ex); throw new Exception("NHibernate initialization failed", ex); } } public static ISession OpenSession() { return SessionFactory.OpenSession(); } }
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The ISessionFactory is bound to a static (and readonly) variable B. All your threads can share this one constant, because the ISessionFactory implementation is
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Architectural patterns for persistence
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thread-safe. This session factory is created in a static constructor C, and this constructor is executed the first time this helper class is accessed. The ISessionFactory is built from a Configuration D; this is the same process we ve demonstrated throughout the book. You catch and log exceptions E, but of course you should use your own logging mechanism rather than Console.Error. The utility class has one public method: a factory method for new ISessions F. It s a convenient method to shorten the code required for the most common usage of this class: opening new sessions. This (trivial) implementation stores the ISessionFactory in a static variable. Note that this design is completely cluster-safe. The ISessionFactory implementation is essentially stateless (it keeps no state relative to running transactions), except for the second-level cache. It s the responsibility of the cache provider to maintain cache consistency across a cluster. Thus you can safely have as many ISessionFactory instances as you like. Despite this freedom, in practice you want as few as possible, because the ISessionFactory consumes significant resources and is expensive to initialize. Now that we ve solved the problem of where to put the ISessionFactory instance (a common question that arises with NHibernate newcomers), we ll continue with the use-case implementation.
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In this section, you ll write the code that implements the place bid use case in a single PlaceBidForItem() method, shown in listing 10.2. This code can live in an ASP.NET code-behind or a function in a console application. It doesn t matter; let s assume that wherever it is, the containing program will get some user input and pass it to this method. It s worth noting that this code sample isn t considered a good implementation, but we ll get to that shortly. It does give us a nice starting point for demonstrating the varying degrees of separation you can introduce into your applications.
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